1. Field of the Invention
The present invention relates generally to a process for removing sulfate anions from sulfate-contaminated waste water and, more particularly, to a process for removing sulfate anions from sulfate-contaminated waste water utilizing liquid emulsion membranes.
2. Description of the Prior Art
It is well recognized that a process for removing sulfate in anion form from waste water would be very beneficial to both the environmental and business interests of this country. Presently, thousands of sites on federal and private lands contain sulfate-contaminated water. For example, acid mine drainage (AMD) from many active and inactive mines produce low-pH water that contains sulfate levels higher than discharge standards such as the Environmental Protection Agency Drinking Water Standards.
Processes do presently exist for removing sulfate in anion form from solutions. However, most of these known and proposed processes involve the precipitation of a compound containing the sulfate anion that itself has a low solubility in water. Examples of this process utilize calcium and barium compounds. Unfortunately, the solubility of calcium sulfate in many waste waters results in a level of sulfate that is above that allowed in the Environmental Protection Agency's (EPA's) current Drinking Water Standards, which is 0.25 g/L SO.sub.4. Barium, on the other hand, is capable of reducing the level of sulfate to very low levels. However, most barium-containing compounds are known to be very toxic. Therefore, the use of calcium and barium in "general" waste water treatment approaches has been considered by those skilled in the art to be risky. In addition, most barium-containing compounds capable of being used in a sulfate removal process are considered to be overly expensive and therefore not desired for use in a commercial process of sulfate removal.
A more recent technique that has been proposed for use as a sulfate reducer in waste water treatment uses sulfate reducing bacteria. In general, this process uses bacteria that have the ability to break down sulfate through their biological activity into compounds such as hydrogen sulfide. Some studies have indicated that this technique may be relatively expensive and some data indicates that it may not be effective enough in some situations to meet the EPA's discharge standards. Another concern with this process is that hydrogen sulfide gas is generated as a product of the sulfate reduction. Since this is a toxic gas which must be captured for use or storage, special considerations must be given to the design and operation of waste water plants using this technique. This requirement will obviously add to the capital and operating costs of the process, and will also unduly complicate the safety aspects of the operation. One of the proposed uses of the hydrogen sulfide gas generated during this process is the precipitation of metals from the waste water. However, this adds an additional potential complication to the process because the required amount of hydrogen sulfide for metal precipitation may not be matched by the production from the sulfate reduction process.
Solid membrane techniques such as reverse osmosis (RO) have also been proposed for use in the general treatment of waste water. However, in its application to sulfate removal from waste waters, the RO process will produce two streams. One stream is a relatively concentrated solution containing most of the sulfate and the other stream is a relatively pure solution. The partially concentrated sulfate solution will require further processing (concentration) before the sulfate can be disposed of. In addition, the RO technique is relatively non-specific. Therefore, essentially all of the impurities contained in the original waste water will stay in the resulting concentrated stream with the sulfate. These other impurities may make the final sulfate material more toxic, and thus more difficult to dispose of. Other disadvantages of the RO process in this application are that fouling of the membrane with solids is likely when treating acid mine drainage waters. In many mine drainage water treatment situations, the water will first be treated with lime to precipitate out most of the contained metals, such as iron. Thus, the waters to be treated subsequently for sulfate removal will be saturated with calcium sulfate which tends to continue to precipitate-out of solution for extended periods of time. Additionally, solid membrane techniques like RO are relatively expensive approaches for the treatment of waste waters such as acid mine drainage waters which in many cases amounts to very large flow rates.
As seen from the foregoing, although different processes have been considered for removing sulfate from waste water streams they are not without their shortcomings. Consequently, there is a need for an improved process for removing sulfates in anion form from waste water streams which can easily reduce the level of sulfate in the waste water to levels well below the EPA's Drinking Water Standards. The process must be not only effective in reducing the level of sulfate in the waste water to an environmentally acceptable level, but in addition the process must avoid the use of, or production of, highly toxic materials during process implementation. Finally, the process must be capable of operating without adding any extraneous materials to the waste water that would adversely effect water quality.